Method of surface-finishing stainless steel after descaling

ABSTRACT

A new surface finishing process for stainless steel where beautiful, bright and milky white coloured surface is obtainable even for such grades as high carbon containing 13 chromium steel and high sulfur containing free cutting stainless steel is disclosed. After removing the surface scale, (1) immerse the stainless steel into the 1st treating solution containing nitric acid: 5-40 g/l, hydrofluoric acid: 2-10 g/l and Fe(III) ion: 15-40 g/l for 5-180 sec., then rinse in water, (2) and successesively immerse into the 2nd treating solution containing nitric acid: 120-250 g/l, Fe(III) ion: 15-40 g/l for 30-300 sec., then rinse in water.

FIELD OF THE INVENTION

This invention relates to a surface finishing process for stainlesssteel being carried out after removal of its surface scale formed in hotworking and/or heat treatment process.

BACKGROUND ART

A surface scale being formed on the stainless steel in hot workingand/or heat treatment process is removed in a descaling line byimmersing, for example, into a sulfuric acid or a hydrochloric acidsolution, or in a supplemental acid solution after being treated in asalt bath. And then, a surface finishing process has been furthercarried out in order to give a corrosion resistance or brightness to thesurface.

And the process of immersing into the nitric-hydrofluoric acid solutionor into the nitric acid solution has been carried out conventionally forthe surface finishing process. However, for such grade of stainlesssteel as, for example, low chromium containing ferrite stainless steel,high carbon containing martensite stainless steel or sulfur containingfree cutting stainless steel, those conventional processes have beenshowing problems of giving a yellowish, greyish or spotted colour to thesurface and to give an inferior brightness to the surface of thestainless steel.

The purpose of this invention is to provide a novel surface finishingprocess where yellowish, greyish or spotted colour will not arise on thesurface of the stainless steel. Namely, the purpose of this invention isto provide a new surface finishing process being carried out after thedescaling process, wherein a beautiful, bright and milky white colouredsurface can be obtained even for such stainless steel as high carboncontaining 13 chromium steel (JIS SUS440C etc) and high sulphurcontaining 13 chromium steel (JIS SUS416, SUS420F etc), instead of theconventional surface finishing process where rough, yellowish andgreyish coloured surface has been resulted.

DISCLOSURE OF THE INVENTION

This invention is a surface finishing process for stainless steel beingcarried out after removal of the surface scale formed in hot workingand/or heat treatment process, wherein (1) immerse the stainless into a1st treating solution containing nitric acid: 5-40 g/l, hydrofluoricacid: 2-10 g/l and Fe(III) ion: 15-40 g/l for 5-180 sec., then waterrinsed, (2) and successively immerse into a 2nd treating solutioncontaining nitric acid: 120-250 g/l and Fe(III) ion: 15-40 g/l for30-300 sec.

The nitric acid concentration in the 1st treating solution of thisinvention is 5-40 g/l. In order to slightly dissolve and make smooth thedescaled rough surface of the stainless steel, pH value is preferable tobe kept less than 1.00. And when the concentration of the nitric acid inthe 1st treating solution is less than 5 g/l, it becomes not easy tokeep the pH value to be less than 1.00. However, when it is over 40 g/l,the dissolving of the surface is excessively accerelated.

The concentration of hydrofluoric acid in the 1st treating solution ofthis invention is 2-10 g/l. When it is less than 2 g/l, the dissolvingof the surface may not be promoted for a material of high corrosionresistance. However, when it is over 10 g/l, the dissolving of thesurface is excessively promoted for a material of low corrosionresistance such as JIS SUS430 or 440C.

The concentration of Fe(III) ion in the 1st treating solution of thisinvention is 15-40 g/l. Fe(III) ion may react with undissociatedhydrofluoric acid and effectively may keep the amount of theundissociated hydrofluoric acid in the solution. When it is less than 15g/l, the reactive power of above may be too weak. However, when it isover 40 g/l, crystallized iron fluoride compounds may appear and causeproblems.

After immersing into the 1st treating solution, water rinsing shall becarried out thoroughly in this invention. In the treatment of immersinginto the 1st treating solution, small sized smut may appear on thesurface of the stainless steel, and these smut have to be removedthoroughly in this water rinsing. By this water rinsing, effects broughtby the 2nd treating solution may be enhanced, and the more beautifulsurface of the stainless steel becomes obtainable. Hot water rinsing maybe used in this water rinsing.

The concentration of nitric acid in the 2nd treating solution of thisinvention is 120-250 g/l. When it is less than 120 g/l, chemicalreaction with evolving hydrogen may appear and the dissolving of thesurface of the stainless steel may be activated and accerelated for lowchromium containing stainless steel. In case of increased concentrationof the nitric acid, an oxydizing reaction brought by the nitric acid maybecome more enhanced and the surface of the stainless steel tends tobecome passivated. However, when it is over 250 g/l, it may change tobecome more activated and may strongly dissolve the stainless steel withvigorous evolution of NO_(x) gas, which leads the surface of thestainless steel to be rough and blackish grey colour.

The concentration of Fe(III) ion in the 2nd treating solution of thisinvention is 15-40 g/l. As precisely explained later, the brightness ofthe surface of the stainless steel is enhanced in this invention byrepeating alternate proceedings of slightly dissolve the surface andpassivate it, and Fe(III) ion may keep continue these repeating ofalternate reactions. When the concentration of Fe(III) ion is less than15 g/l, the effect to keep continue the reaction becomes insufficient.It may be allowed to be over 40 g/l, however, it is economicallyunsuitable. The concentration of around 25 g/l may be preferable inoperation. Water rinsing shall be carried out after the step ofimmersing into the 2nd treating solution. Hot water rinsing isapplicable instead of water rinsing.

According to this invention where both treatments of using the 1sttreating solution and the 2nd treating solution of this invention arecarried out, stainless steel having the surface of bright and milkywhite colour may be obtained. And these stainless steel are preferablesince their bright and milky white coloured surface are fine and smooth,and further they are sufficiently passivated.

In this invention, the duration of immersing in the 1st treatingsolution is 5-180 sec, and the duration of immersing in the 2nd treatingsolution is 30-300 sec. Strictly speaking, the most preferable durationin the 1st treating solution as well as that in the 2nd treatingsolution may vary depending on a grade of stainless steel to be treated.However, the most preferable duration may easily be decided by carryingout in advance a immersing test within the scoupe of the above explainedduration. Namely, the most preferable duration may easily be determinedby referring the result of the immersing test where bright and milkywhite coloured surface is obtained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1: General explaining chart showing active state-passive state ofstainless steel.

FIG. 2: Explaining chart of electric potential of stainless steelimmersed in 1st treating solution of embodiment example and ofcomparative example.

FIG. 3: Explaining chart of electric potential of stainless steelimmersed in 2nd treating solution of embodiment example and ofcomparative example.

MODE FOR CARRYING OUT THE INVENTION

The inventors provided hot rolled and heat treated wire rod of freecutting 13% chromium steel (JIS SUS420F) containing 0.35% sulfur and of7 mm diameter as specimen. Specimens were subjected to the finishingprocesses shown in Table 1 after being descaled by sulfuric acidpickling→salt bath immersion→nitric-hydrofluoric acid pickling.

No. 1-7 in Table 1 are embodiment examples of this invention where the1st treating solution and the 2nd treating solution of this inventionwere used. Wherein water rinsing were thoroughly carried out in betweenthe treatment by 1st treating solution and the 2nd treating solution,and also after the treatment by the 2nd treating solution. No. 8-14 inTable 1 are comparative examples where the 1st treating solution was aconventional high concentrated nitric-hydrofluoric acid solution and the2nd treating solution was a conventional nitric acid solution withoutany addition of Fe(III) ion. Water rinsing were carried out thoroughlyalso in comparative examples between the 1st treating solution and the2, and after the 2nd treating solution.

As shown in the column of surface condition on 1st treating solution inTable 1, the 1st treating solution of No. 1-7 of this invention did notexcessively attack the surface of the stainless steel. Namely, thesurface of the stainless steel after being treated by the 1st treatingsolution of the invention was showing a mixed colour of greyish andwhite or a mixed colour of blackish and white as shown in mark Δ, andany sign of being severely attacked was not observed. Whereas, the 1sttreating solution of No. 8-14 of comparative example excessivelyattacked the surface of the stainless steel and was showing blackishcoloured surface as shown in mark x in Table 1.

As shown in the column of surface condition on 2nd treating solution inTable 1, all of the 2nd treating solution of No. 1-7 of this inventionbrought a bright and milky white coloured surface. These are showingthat the smoothing of the surface of the stainless steel was resulted byrepeating the alternate proceedings of slightly dissolving the surfaceof the stainless steel and then passivating it. Whereas, surfacecondition of comparative example of No. 8-14 was blackish coloured andshowing that their smoothing effects had been insufficient.

Study of the electric potential of metal referring to the standardhydrogen electrode has been carried out widely in order to investigate acorrosive property of the metal in acid solution. The inventors measuredelectric potential on specimens by using collated Ag—AgCl electrode.FIG. 1 is a general explanation chart of a electric potential of astainless steel showing an active region and passive region. Thehorizontal axis shows an electric current dencity being correspoding toa dissolving speed of corrosion. And the vertical axis shows an electricpotential of metal where the more large value in the plus side shows themore strong oxidizing property of the acid solution. In this figure,curved line α represents a dissolving curve of the stainless steel,{circle over (1)}-incurved line β, {circle over (2)}-incurved line β and{circle over (3)}-in curved line β are representing reducing reactionsof oxidizing components (nitric acid ion or Fe(III) ion) in the acidsolution and curve γ is representing a reducing reaction of hydrogenion. And the surface condition of stainless steel may be defined by thelocation of and the reaction degree at the crossing point of thedissolving curved line and reducing reaction curved lines.

When, for example, 13% chromium containing stainless steel (SUS 420J2)is immersed into the 1st treating solution being containing nitric acid:40 g/l and HF: 10 g/l, the electric potential of the stainless steel isin the active region in FIG. 1 and the surface is dissolved. When thestainless steel is then immersed into the 2nd treating solution of beingonly comprised of nitric acid after the water rinsing, the electricpotential comes to the plus region and the stainless steel ispassivated. However, when the stainless steel is immersed into the 2ndtreating solution being containing nitric acid: 200 g/l and Fe(III) ion:25 g/l instead of being immersed into the 2nd treating solution of beingonly comprised of nitric acid, the electric potential vibrates betweenthe plus region and minus region in early stage, but it becomes stablein a certain plus value after repeating this vibration, and thestainless steel becomes as being passivated. In this invention, thesmoothing of the surface of the stainless steel is carried out byapplying this vibrating behavior. Namely, the slight dissolving of thesurface proceeds by being brought its electric potential in the minusregion, and the passivation as well as a desmutting of the surfaceproceeds by being brought its electric potential into the plus region.Thus, in this invention, the smooth and beautiful surface of thestainless steel can be obtained by making to repeat the alternatereaction of slight dissolving and passivation of the surface of thestainless steel.

The inventors immersed specimen into the 1st treating solution of No.1-14 respectively, and provided an electric cell by accompanying of thespecimen as one electrode and the collated Ag—AgCl electrode as theother electrode. And measured respectively the electric potential of thespecimen by using an usual potentiometer. The temperature of thesolution and the duration of immersion are shown in Table 1. FIG. 2shows a result obtained by this measurements.

In FIG. 2, the electric potential of the specimen being immersed in the1st treating solution of this invention comes to be in minus region,however, the curved line in the minus region is in a gentle slope andthe specimen shows smooth and greyish coloured surface. On the otherhand, in case of the 1st treating solution of comparative examples, theelectric potential sharply drops in the minus region just after theimmersion and shows excessive drop and vigorous corrosive reaction. Inthis case, thick smut are formed and shows rough and blackish colouredsurface.

All specimens being treated in the 1st treating solution were waterrinsed thoroughly, and then immersed respectively into the 2nd treatingsolutions being shown in Table 1. FIG. 3 shows the result of themeasurements of the electric potential in the 2nd treating solutions. Inthe 2nd treating solution of the comparative example, the electricalpotential sharply drops into the minus region in a short time and thenbecomes flat almost at the same level.

This shows an active state where the surface is not passivated and thecorrosion of the specimen continues to proceed accompanying the hydrogenevolving reaction. And the blackish coloured surface are resulted at theend of the proceeding. On the other hand, in case of 2nd treatingsolution of this invention, the electric potential drops once into theminus region but moves into the plus region in a short time, and finallyare kept flat in the plus region after repeating alternate change ofthis electric potential. Thus, in case of the 2nd treating solution ofthis invention, the active dissolution in the minus region and thepassivation in the plus region are repeated, and the surface becomessmooth and milky white colour as the result of repeating of slightdissolution and passivation.

The inventors had carried out further the same study as shown in Table1, FIG. 2 and FIG. 3 on other grades of stainless steel than JISSUS420F, for example, on JIS SUS416, SUS420J2 and SUS440C where surfaceof those were greyish or blackish grey colour after the finishingprocess, and obtained the same results as explained.

Industrial Applicability of the Invention

This invention enbables to obtain a sufficiently smooth and bright milkywhite coloured surface being preferred by the consumers for such gradesof stainless steel as JIS SUS416, SUS420F, SUS420 and SUS440C byreplacing the conventional finishing process where insufficiently smoothand blackish coloured surface has been resulted. TBALE 1 1st treatingsolution 2nd treating solution 30° C., immerse for 60 sec. 30° C.,immerse for 180 sec. HNO₃ HF Fe³⁺ surface HNO₃ Fe³⁺ surface No. wt % wt% wt % condition wt % wt % condition 1 35 10 35 Δ 130 15 ◯ 2 35 10 35 Δ130 15 ◯ 3 35 10 25 Δ 130 25 ◯ 4 35 10 25 Δ 200 15 ◯ 5 35 5 25 Δ 200 25◯ 6 10 10 25 Δ 200 25 ◯ 7 10 5 25 Δ 240 35 ◯ 8 80 20 — X 100 <15 X 9 8020 — X 200 <15 X 10 60 10 — X 100 <15 X 11 60 10 — X 200 <15 X 12 80 20— X 80 <15 X 13 80 20 — X 250 <15 X 14 60 10 — X 250 <1.5 Xsurface condition◯: milky white colourΔ: greyish coloure or spotted colour of white and blackishX: blackish colour

1. A surface finishing process for stainless steel being carried outafter removing the surface scale formed in hot working and/or heattreatment process, wherein 1) immerse the stainless steel into a 1sttreating solution containing nitric acid: 5-40 g/l, hydrofluoric acid:2-10 g/l and Fe(III) ion: 15-40 g/l for 5-180 sec., then rinse in water,2) and successively immerse into a 2nd treating solution containingnitric acid: 120-250 g/l and Fe(III) ion: 15-40 g/l for 30-300 sec.,then rise in water.